This invention relates to an apparatus and a method for controlling the rotational speed of an internal combustion engine mounted on a vehicle mainly during an idling operation, and more particularly relates to an apparatus and a method to control the rotational speed of the engine by limiting a value of a control signal which is varied to manipulate or control the amount of intake air of the engine.
Requirements have recently arisen for precise control of the rotational speed of an internal combustion engine even during idling periods in order to eliminate harmful components in the exhaust gases and to reduce unnecessary fuel consumption. Such requirements have led to the development of a feedback control system for controlling the amount of air entering the engine so as to bring the actual rotational speed of the engine closer to a desired value determined in accordance with engine operating conditions, such as temperature, for example. In such a system, when the actual rotational speed is below the desired value, the speed is increased by increasing the amount of intake air. However, this approach often causes problems as follows. If a brake is applied to slow down a vehicle while running with the transmission gear in a position other than the neutral position, the rotational speed of the engine is decreased with the decrease of the vehicle speed because the engine speed is restrained by the vehicle speed. Under such conditions the feedback control system operates to increase the amount of intake air so as to increase the actual rotational speed of the engine. However the actual rotational speed can not be increased because of the restraint of the vehicle speed and therefore the feedback control system operates to further increase the intake air amount. This sequence leads to an increase of the driving power or output power of the engine and a consequent reduction of the braking effect. Accordingly there is an unwanted or undesirable increase in the stopping distance necessary to slow down a running vehicle and to bring it to rest, as well as in brake pedal pressure necessary to attain the braking effect.